Whisky or whiskey is taken from the anglicization of the Gaelic word for water, uisce or uisge. This word became applied to distilled spirits as it was known is Latin as aqua vitae, literally water of life.
Now on the spelling difference, there doesn’t seem to be too much behind it beyond a regional preference, and that it also tended to correlate with origins and local styles of spirits. The spelling of whiskey is most common in the United States and Ireland, with the spelling whisky used for spirits produced in the rest of the world. Certain whiskeys common to America tend to use this style, such as bourbon whiskey, while Scotch and Japanese whiskys use the other spelling.
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